drm/xe: Use a NULL pointer instead of 0.

The last argument of xe_pcode_read() is a pointer. Use NULL instead of 0.

Fixes: 92d44a42 ("drm/xe/hwmon: Expose card reactive critical power")
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Francois Dugast <francois.dugast@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240117134048.165425-6-thomas.hellstrom@linux.intel.com